Recent findings in Drosophila have shown that 1) a significant proportion of spontaneous mutations are caused by insertions of mobile genetic elements, and 2) certain genetic suppressor systems are mediated through insertions of specific mobile elements. We are investigating the molecular mechanism of one such suppressor system: recessive mutations at the suppressor-of-sable [su(s)] locus supress recessive mutations at the vermilion (v) locus that are caused by insertions of the mobile elements 412 and B104. DNA sequences of su(s) have been cloned and are being characterized. The insertions of foreign DNA that are associated with 14 su(s) mutant alleles have been localized to a region of DNA that encodes the 5' end of the message. The DNA sequences that give rise to the about 5 kb poly A+ su(s) message consist of at least 5 exons interspersed over 8 kb of genomic DNA. Coding sequences from the two largest exons will be ligated into an expression vector to produce a fusion protein, against which antibodies can be produced. Antibodies against the su(s) portion of the fusion protein will be recovered and used as probes to identify the location and function of the su(s) protein product within the organism. The interaction of the su(s) protein with the v locus will be studied to determine how suppression is effected. By gaining an understanding of this phenomenon, we will learn if this type of suppression mechanism is an adaptive feature of Drosophila to deal with mutations caused by mobile element insertions.